Chemical Mechanical Planarization or Polishing, commonly referred to as CMP, is a method of planarizing or polishing a semiconductor wafer or other type of substrate. A typical CMP apparatus includes a platen having a polishing pad thereon, a polishing head for holding the substrate thereon, and a mechanism for providing relative movement between the polishing head and the pad. Referring to FIG. 1, the polishing head 12 includes a carrier 14 having a subcarrier 16 with a lower surface 18 for pressing the substrate 20 against the polishing pad (not shown) during the polishing operation, and a retaining ring 22 circumferentially disposed about the subcarrier. The retaining ring 22 generally restrains or limits lateral movement of the substrate 20 relative to the subcarrier 16 to hold or retain the substrate between the subcarrier and the polishing pad. Generally, the polishing head 12 further includes a backing ring 24 through which a force may be applied to the retaining ring 22, and chambers 26, 28, above the subcarrier 16 and the backing ring 24 respectively that may be pressurized to urge or force the retaining ring 22 and the subcarrier 16, with the substrate 20 thereon, against the polishing pad. Typically, the chambers 26, 28 above the backing ring 24 and subcarrier 16 are separate so that the force applied to the substrate 20 and to the retaining ring 22 can be controlled independently.
Planarizing or polishing a surface of a semiconductor substrate, for example, between certain processing steps allows more circuit layers to be built vertically onto a device. However, as feature size decreases, density increases, and the size of substrates increase, CMP process requirements become more stringent. Substrate to substrate process uniformity as well as uniformity of planarization across the surface of a substrate are important issues from the standpoint of producing semiconductor products at a low cost. As the size of structures or features on the substrate surface have been reduced to smaller and smaller sizes, now typically about 0.2 microns, the problems associated with non-uniform planarization have increased. This problem is sometimes referred to as a Within Wafer Non-Uniformity (WIWNU) problem.
Many reasons are known in the art to contribute to non-uniformity problems. These include edge effect non-uniformities arising from the typically different interaction between the polishing pad at the edge of the substrate 20 as compared to at the central region. Typically, more material is removed from the edge of the substrate 20 than at the center. That is the edge of the substrate 20 is over polished. This is known as the edge effect. Many attempts have been made in the art to correct or compensate for the edge effect. However, efforts to solve this problem have not heretofore been completely successful.
One approach in an attempt to correct this over polishing of the edge of the substrate 20, has been to apply a somewhat higher force to the retaining ring 22 than to the subcarrier 16. The polishing pad under the retaining ring 22 is deformed or compressed with the effect that the force between the surface of the polishing pad and the surface of the substrate 20 near its edge is reduced. This results in less material being removed from the surface of the substrate 20 near its edge.
While an improvement over earlier designs, this approach is not an entirely satisfactory solution. One problem with this approach, graphically illustrated in FIG. 2, is that as the polishing pad deforms under pressure of the retaining ring 22 during the polishing operation, it is pulled away from the surface of the outer edge of the substrate 20 adjacent to an inner edge of the retaining ring 24. Thus, the approach described above can change the situation from one in which too much material is removed from the surface of the substrate 20 near its edge to one in which too little is removed. The usual method of controlling this rebound effect, as it is commonly known, is to attempt to limit pad deformation by achieving an exacting balance between force applied to the subcarrier 16 and the retaining ring 22. That is as the force applied to the retaining ring 22 rises, the force applied to the subcarrier 16 is also increased. When properly balanced, both the size of the area near the edge of the substrate 20 separated from the polishing pad, rebound effect, and over polishing of the surface near the edge of the substrate, edge effect, is reduced. However, achieving and maintaining such an exacting balance can be extremely difficult given the changes in polishing pad thickness and properties likely to occur over time. Achieving such a balance can be impossible where the pad deformation exceeds that which can be compensated for within the limits of an available range of force that can be applied to the subcarrier 16 and the retaining ring 22, or within the limits of force that can be applied to the substrate 20. This is particularly a problem with the latest generation of polishing pads using materials having viscoelastic properties, such as polyurethane, commercially available from RODEL of Newark Del. By viscoelastic it is meant the material of the polishing pad exhibits different elastic properties to force applied in different directions, or for different lengths of time.
Another prior art approach is to provide harder polishing pads less susceptible to deformation. This approach however is often neither possible nor desirable for a number of reasons. In the first place, some limited amount of deformation is necessary to prevent excess removal of material near the edge of the surface of the substrate 20, therefore using a harder, less compliant material for the pad would diminish the benefit of using a retaining ring 22. Moreover, using a harder, less compliant material for the polishing pad would decrease deformation of the polishing pad, could actually increase the rebound effect since the harder material, being less flexible, would take a greater time to recover from the deformation. Thus, for a polishing head 12 moving at a given speed over the polishing pad, the distance between the inner edge of the retaining ring 22 and the point at which the polishing pad has rebounded sufficiently to touch the surface of the substrate 20 would increase for a harder polishing pad.
Accordingly, there is a need for a CMP apparatus and method that reduces if not eliminates excess removal of material from the surface near the edge of a substrate (that is reducing the edge effect) while also reducing the area near the edge of the substrate 20 separated from the polishing pad (that is reducing the rebound effect).
Another problem with conventional retaining rings 22 arises from the fact that they are consumable items, having a lower surface 30 from which a thin layer of material is removed during the polishing operation. Moreover, as shown in FIG. 3 retaining rings 22 are often made of a polycrystalline ceramic material that includes a number of partial crystals 32 along the lower surface 30. Partial crystals 32 are created by machining a flat surface on a molded retaining ring core, thereby creating a surface that generally includes many partial crystals. These partial crystals 32 are held in place by mechanically interlocking with other surrounding whole crystals 34 and partial crystals. As the retaining ring 22 wears from the original lower surface 30 to that represented by line 36 in FIG. 3, the mechanical interlocking can be destroyed as a result of the wear that occurs during one or a succession of polishing operations, allowing partial crystals 32 or chips of ceramic material to become dislodged and trapped between the substrate 20 and the polishing pad during a polishing operation. This in turn can damage the surface of the substrate 20, rendering it completely worthless. A loss, depending on the point in processing, of up to several thousand or even tens of thousands of dollars. In CMP apparatus having multiple heads 12, several substrates (e.g., wafers) may be lost.
Many attempts have been made in the prior art to solve this problem, including manufacturing retaining rings 22 out of metal. However, metal has proven to be generally unsuitable for retaining rings 22 for a number of reasons. In the field of semiconductor manufacturing, metal is undesirable due to the possibility of metal contamination of the substrate 20 by material removed from the retaining ring 22 during the polishing operation. Moreover, it is generally desirable that some material be removed from the lower surface 30 of the retaining ring 22 during the polishing operation to maintain a highly planar surface on the retaining ring without which the WIWNU might be increased. For a further explanation of the effect of a non-planar retaining ring surface on the WIWNU refer to commonly assigned, co-pending U.S. patent application Ser. No. 09/652,855 filed Aug. 31, 2000 and entitled Chemical Mechanical Polishing Apparatus and Method Having a Rotating Retaining Ring, which is incorporated herein by reference. The negligible removal rate of material from the lower surface 30 of a metal retaining ring 22, might inhibit this conditioning from occurring. In addition, because retaining rings are considered consumable items, the expense of providing an initially highly planar lower surface 30 on a metal retaining ring 22 would add significantly to operating costs of the CMP apparatus.
An attempt has also been made to solve this problem, by making retaining rings 22 out of Techtron®. Techtron® is a plastic, commercially available from DSM Engineering Plastic Products, of Reading, Pa. Because it is a plastic, retaining rings 22 constructed of this material avoid the chipping problem of ceramic rings and the potential contamination of metal retaining rings. However, retaining rings 22 made of Techtron® exhibit excessive and rapid wear leading to a lower useful lifetime for the retaining ring. This is undesirable since, in addition to the expense of the retaining ring 22 itself, replacing it generally involves a considerable amount of equipment downtime to (i) run-in or season the new retaining ring, and (ii) to characterize and/or set process parameters with the new retaining ring. Challenges in setting the process may involve changing rotation speed, pressure, time and the like.
Therefore, there remains a need for a CMP apparatus and method that reduces if not eliminates excess removal of material from the surface near the edge of a substrate, referred to as edge effect, while also reducing the area near the edge of the substrate separated from the polishing pad, referred to as rebound effect. There is also a need for a retaining ring that avoids the chipping or spalling problem of ceramic retaining rings and the potential contamination of metal retaining rings, while providing an acceptable useful life.